Applications of thin, transparent and electrically conducting films are numerous. The most attractive application is as a transparent electrode for low-cost photovoltaics and other opto-electronic applications. Light emitting devices often require such electrodes, in particular, large area displays. Currently, the existing technology uses conducting metal oxide films, primarily indium-tin oxide (ITO) and doped zinc oxide for these applications. These films have a limited transparency/conductivity trade-off and are produced by expensive vacuum deposition techniques. Such films are also hard and brittle and may therefore be unsuitable for flexible coatings such as plastic electronics. A flexible alternative that has been considered is a film of a conducting polymer, but such films are much less conductive and more sensitive to radiation and chemical attack and would thus not be good candidates for ITO replacement.
In recent years, there is a growing interest in finding alternatives for these transparent oxide electrodes. The primary candidates have been carbon nanotube-based electrodes. However, such films could not exceed the performance of ITO films in terms of conductivity vs. visible light transmittance. There are several problems in producing highly conductive, transparent carbon nanotube mesh films. The limited solubility of the tubes makes it difficult to disperse them in various solvents for efficient coating applications. To produce such dispersions, high molecular weight polymer surfactants are required, which produce an insulating or semiconducting layer [1] around the nanotubes and thus significantly increase the inter-tube contact resistance.
Another alternative is to use pure carbon nanotube meshes or fabrics for this purpose, but here the nanotube density is too high and optical transmission is degraded, and it is thus difficult to integrate such meshes into thin-film devices. The same difficulties hold for other types of prefabricated nanowires made of various oxides and semiconductors.
Recently, thin conducting films consisting of high aspect ratio nanostructures have been suggested as a substitute for metal oxide based transparent electrodes, particularly in combination with conducting polymer based devices [2, 3, 4]. Such films, made of metallic nanowires have high conductivity while maintaining a metal volume fraction as low as ˜1% and thus are highly transparent.
Many schemes of synthesizing conducting and semi-conducting nanowires were developed in the last 15 years. A very high control level of the nanowires geometry and composition, including modulation of compositions along or across the nanowires, has been achieved. A control over the position and orientation of the nanowire growth has been achieved in catalytic growth of carbon nanotubes and semiconductor nanowires by positioning of the catalyst particles at selected places. However, the task of forming uniform thin films of such elongated nano-objects to obtain highly conductive meshes over large areas has been more difficult to achieve. High molecular weight polymeric surfactants are required for dispersing the nanowires/nanotubes in various solvents. These polymers usually form insulating barriers over the nanowires, which would then require annealing to reduce inter-wire electrical resistance [2], unless the polymer itself is (semi-)conducting [1, 4].
Peumans et al., have recently published a first calculation and demonstration of a random silver nanorods mesh electrode as a replacement for a metal oxide film in a polymer based solar cell [2]. Peumans et al used prefabricated silver nanorods with an average aspect ratio of ˜84, coated by a high molecular weight polymer and dispersed in a solvent to prepare the thin conductive film. The film required substantial annealing to reduce the contact resistance between the nanorods, which probably was the primary factor limiting the performance of this film. The film, with comparable transmittance and conductivity to an ITO film, exhibited 19% higher photocurrent when used for the polymer solar cell compared to the ITO analog.
Gold nanowires have also been prepared in oleylamine, employing a variety of methods.